Low EMI keypad circuitry

摘要

Disclosed is keypad circuitry operable to detect a pressed key while reducing electromagnetic interference (EMI). The keypad circuitry is operable to reduce EMI in two ways: a) reducing the voltage swing occurring at the row circuitry and column circuitry of the keypad, and b) reducing the number of signal transitions by restricting the signal transitions to occurring at the column and row corresponding to a pressed key. By limiting signal transitions to occurring only at the row and column corresponding to a pressed key, fewer signal transitions occur, and thus, less EMI is produced. Additionally, reduced voltage swings at the row circuitry and column circuitry results in reduced EMI.

主权项

1. Keypad circuitry comprising:
a plurality of first signal lines; a plurality of second signal lines crossing the plurality of first signal lines; a plurality of keys, each key configured to connect one of the first signal lines to one of the second signal lines; a clamping circuit coupled to each of the first signal lines and configured to apply, when one of the plurality of keys is pressed, a clamped voltage to the first and second signal lines connected by the key press, wherein during a non-scanning mode, a voltage at the connected first signal line transitions from a first voltage to the clamped voltage; and a drive circuit coupled to each of the first signal lines, wherein during a scanning mode, the drive circuit drives the voltage at the connected first signal line from the clamped voltage to the first voltage.